Objective: The sub spine region is a potential source for both bony and soft tissue impingement around the hip. The primary objective of this study was to observe and classify the variation in morphology of the sub-spine region in patients undergoing hip arthroscopy for the management of femoro-acetabular impingement (FAI). The secondary objective was to evaluate possible pathology associated with sub spine impingement.

Method: A prospective, consecutive series of 95 hip arthroscopies (72 patients) was undertaken; there were 60 males and 12 females (ratio 5:1) with an average age of 29 (range 17 – 50). The bony acetabular rim was exposed utilising a labral cuff reflection technique permitting clear visualisation of the sub spine region of the acetabular wall as it extended proximally to the base of the AIIS. The morphology of this region was assessed and classified into one of four subtypes; Type 1 (Concave or flat), Type 2 (Convex), Type 3 (Protrusion) and Type 4 (Sub-articular).

Results: In 48.4% of cases (46 hips) a smooth, concave or flattened surface, extending from the base of the AIIS to the acetabular rim, was observed and considered to be normal (Type 1); In 11.5% of cases, hypertrophy of the sub-spine region was noted which was convex in shape and extended to the level of the brim but not below (Type 2); 26% of cases demonstrated a variably sized localised prominence of the sub-spine region which was distinctly separate to the rim and extending antero-inferiorly (Type 3); 14.6% of cases demonstrated hypertrophy of the sub-spine region which extended distally below the level of the acetabular rim (Type 4); morphological subtypes 2, 3 and 4 were considered to be abnormal.

Bruising of the labrum was observed in 45.8% of cases overall and in 37% of cases with type 1 (normal), 45% with type 2, 52% with type 3 and 64% of cases with type 4. The frequency of labral bruising was more significant with increasing sub spine morphology, from type 1 to type 4 (p<0.05).

Rim fractures were present in 15.8% of all cases however were observed more commonly in cases with abnormal sub-spine morphology (p=0.015): A rim fracture was found in 27.2% of cases with type 2, 20.8% with type 3 and 28.6% with type 4 as opposed to 6.5% of cases with type 1 (normal).

Conclusion: Four distinct patterns of sub-spine morphology were observed in this cohort of patients and were considered abnormal in over half of cases studied, raising the possibility of bony and/or soft tissue impingement in this region. Arthroscopic signs of labral bruising and rim fractures were strongly associated with abnormal sub spine morphology and should alert the surgeon to the potential presence of sub spine impingement. Re-contouring of the sub spine region may be necessary as part of the management of the acetabular rim in patients with FAI, having the potential for reducing bony impingement and increasing the space available for soft tissues in this region.